"When making life-or-death, war-or-peace decisions, a president can't just pop off or lash out irrationally," she said on behalf of the Democratic presidential nominee at La Salle University in Philadelphia. "We need an adult in the White House. I guarantee you."
Obama criticized Trump over his spreading of the birther conspiracy theory, continuing a rebuke she first made earlier this month
while stumping for Clinton in Virginia.
"There are those who questioned and continue to question for the past eight years whether my husband was even born in this country," Obama said to boos. "And let me say: hurtful, deceitful questions deliberately designed to undermine his presidency. Questions that cannot be blamed on others or swept under the rug by an insincere sentence uttered at a press conference."
The first lady denounced the "negativity" and "name-calling" in the election and urged supporters to "choose the right person to lead our country forward."
"So if a candidate is erratic and threatening, if a candidate traffics in prejudice, fear and lies on the campaign trail, if a candidate thinks that not paying taxes makes you smart or that it's good business when people lose their homes, if a candidate regularly and flippantly makes cruel and insulting comments about women -- about how we look, how we act -- well, sadly, that's who that candidate really is," she said.
The first lady was referring to a variety of attack lines Clinton has used to hit Trump in recent days and at Monday's debate, including his 2006 remark that he hoped for a real estate market crash, his criticism of a former Miss Universe's figure and his spreading of the "birther" falsehood.
She continued: "Experience matters. Preparation maters. Temperament matters. And Hillary Clinton has it all. She's the real deal."
Obama is Clinton's most popular campaign surrogate, with polls pegging her favorable rating near 60%. She is being deployed to encourage college students, millennials and the other voters who overwhelmingly supported her husband in 2008 and 2012.
The first lady made her first solo campaign appearance for Clinton earlier this month, and on Tuesday, she appeared in a television ad on behalf of the Clinton campaign. She made a second campaign appearance Wednesday in Pittsburgh.